This application is related to our co-pending U.S. provisional patent application Ser. No. 08/787,457 for "ACTIVE ELECTRONIC DAMPING FOR STEP MOTOR" filed on even date herewith, the entire disclosure of which is fully incorporated herein by reference.
The invention relates generally to drive control techniques for synchronous motors such as plural drive phase step motors. More particularly, the invention relates to a passive electronic damping feature for such motors.
The general design and operation of a step motor, such as a hybrid permanent magnet synchronous step motor, are well-known. A rotary hybrid machine typically includes a cylindrical rotor which is axially magnetized by an embedded permanent magnet. The rotor is further configured with a number of teeth; commonly fifty teeth are used but other quantities can be selected depending on the particular application requirements. The present invention, however, can also be used with other types of synchronous machines and motors, for example, linear step motors.
Step motor designs also include a stator having a number (typically but not limited to eight) of electromagnetic pole faces which are spaced around the rotor at selected intervals. Each stator pole face presents a toothed pattern to the rotor. When the electromagnets are energized, the induced electromagnetic flux adds to and subtracts from the permanent magnet flux at the various pole faces depending on the rotational position of the rotor. This results in an alignment of the rotor teeth with the stator teeth in stable torque equilibrium at whichever pole faces are carrying the most flux across the rotor-stator gap.
Motion is achieved and maintained by continuously sequencing the electromagnet currents so as to move the location of the stable equilibrium in one direction or another. Torque is excitation and permanent magnet flux crossing the toothed interface between the rotor and stator. In a typical two phase motor, the stator electromagnet windings are grouped into two phases (A and B) and are driven by currents that are in temporal quadrature (the phase A and phase B currents are 90.degree. out of phase with respect to each other). If a particular combination of electromagnetic drive currents (excitation state) is maintained in the various windings, the motor will seek and attempt to hold a particular position due to the presence of a stable torque detent. If the excitation state is changed the rotor is urged to a new position by the resultant torque.
Sustained motion is achieved by continuously sequencing the phase currents through a prearranged set of states. In a typical system, one electrical cycle of the phase currents moves the location of the torque equilibrium through an angle corresponding to one tooth interval. In their aggregate effect, the phase currents represent to the motor a reference position input consisting of a desired position plus an instantaneous position feedback offset.
The rotor attempts to seek the reference position (torque equilibrium or net zero torque condition) as if it were attached to the command by a spring which exerts a restoring torque when the rotor is not at the reference position. Motion or position of the rotor can be analogized to and exhibits the dynamics of a moment of inertia attached to the commanded position by a torsion spring. The stiffness of the "spring" is approximately proportional to the peak phase currents. Such a system, when operated in an uncompensated open-loop mode, exhibits a natural resonance that causes the motor to ring violently in response to step commands. The motor is also inordinately responsive to cyclic torque disturbances that occur in the vicinity of this natural resonance frequency.
Although a step motor will have an intrinsic damping ratio arising from native viscous losses within the motor and any external mechanical system attached to the motor shaft, the intrinsic damping ratio is typically very small and negligible in its effect to reduce the natural uncompensated resonance of the motor.
Since the natural resonance of a motor is typically an undesirable operational characteristic, efforts have been directed to increase the damping ratio by either mechanical means or the use of feedback techniques. For example, viscous friction can be mechanically increased in the motor but at the expense of impeding motor velocity and responsiveness. In a typical feedback arrangement, a feedback signal is obtained using an accelerometer mounted on the output shaft. This signal is then used to adjust the command in an attempt at reducing resonant behavior. Such devices are expensive and rather delicate, yet must be mounted to the rotating shaft (moving reference frame) and communicate with electronics on the stator or other stationary reference frame.
The objectives exist, therefore, to provide apparatus and methods for a purely electronic damping technique to effect a virtual viscosity that will damp a step motor's natural resonant response, without the need for external feedback devices, or mechanical loss mechanisms.